A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto a substrate, it has been proposed to use extreme ultraviolet radiation having a wavelength within the range of 10-20 nm, preferably within the range of 13-14 nm.
In order to produce such radiation, a plasma may be produced by focusing a laser at a droplet, thereby changing the droplet, preferably tin droplets, into an extreme ultraviolet radiation producing plasma. Often, a so-called collector mirror may be used to focus the radiation in a focal point.
In addition to extreme ultraviolet radiation, the plasma generally produces debris in the form of particles, such as thermalized atoms, ions, neutrals, nanoclusters, and/or microparticles. The debris may cause damage to the collector mirror and other optics. In order to prevent the debris from causing damage, a buffer gas may be used in the vicinity of the plasma in order to mitigate the debris. Still, it has been found that the collector mirror degrades and deforms when the extreme ultraviolet radiation is being produced.